You need them all. Its your plane that dictates how much of the 3 you get to use. And its up to your skill to make the most of what you get.

If i HAD to pick one attribute that is worth the least amount of attention ... i would say Firepower.

In the Turn&Burn meta its dominated by Spits, Zero's, and certain russians. And they dominate the game as a whole.
Their lots of machingun attacks are weak but they have the maneuverability to stay on target long enough to get the job done.
And that lesson is fairly universal. It doesnt matter if your guns are poop if they cant shoot back because you are better at staying on their six.

And if they are in a hurry about making that damage they can use incendiary ammo and a gunsight to make up for the lack of punch.

Their only limit is how high in the sky they can go and still be dominant. and if the focus of the battle is not where they happen to be they may spend a long enough time repositioning that by the time they get there its too late .

the three most meaningful attributes of both heavy and light fighters to new pilots, in this meta

instead of arguing which is the most important, can we even agree which is the least important of these three?

show your work, please?

For a beginner I would put speed as the least important. Boom'n zoom is not intuitive at the start and on lower tiers does not work too well (for fighters, anyway). So if you are the new guy, you will probably want to be able to put your gun on enemy fast and kill him faster.

Also, since 2.0.5, WG changed the damage model so you can take more damage if youre being shot at from above. This makes roll maneuverability just as important as yaw maneuverability, because you should be hiding your top from potential enemies by rolling your plane over.